This invention relates generally to radio frequency (RF) communication systems and more particularly to spread spectrum communication systems.
As is known in the art, RF communication systems may be broadly classified into two types: A fixed carrier frequency system; and, a spread spectrum system. In a fixed carrier frequency system, each transmitting station is assigned a fixed-in-time, different carrier frequency. If a receiver wishes to receive information transmitted by one of the transmitting stations, the receiver is tuned to the carrier frequency of that transmitting station. While such fixed carrier frequency system is very effective in many applications, in applications where there is noise at the carrier frequency of the transmitter, such noise may "jam" the transmitter's signal thereby preventing a receiver tuned to a "jammed" carrier frequency from obtaining the transmitted information.
Spread spectrum systems are used to reduce the effect of such jamming signals. More particularly, in a spread spectrum system, the carrier frequency of each transmitter is changed over a relatively large bandwidth (i.e., the carrier frequency is "spread" over a relatively large frequency spectrum, (i.e., the so-called "spread spectrum bandwidth"). In a frequency hopping system, the carrier frequency is changed randomly. Typically, the change in carrier frequency is generated by a pseudo-random code generator. A receiver wishing to receive the transmitted signal must have knowledge of the pseudo-random code used by the transmitter (i.e., the "key") and a pseudo-random code generator "matched" to, and synchronized with, the pseudo-random code generator of the transmitter. Thus, the receiver is able to maintain track of, and thereby maintain itself tuned to, the transmitter's pseudo randomly changing carrier frequency. Thus, if the jamming frequency is at a fixed frequency, any individual transmitter is "jammed" for only a small portion of its transmission. By including redundant information in the transmitted signal, any information "corrupted" during a jamming interval may be recovered using various data recovery algorithms. As is also known in the art, in a frequency hopping system, each transmitter has an independent pseudo-random code generator controlling the frequency hopping of its carrier frequency. Thus, with a finite "spread spectrum bandwidth", it follows that there may be times when two transmitters are transmitting with, or approximately, the same carrier frequency, i.e., there are carrier frequency "collisions". Systems which result in frequency collisions are sometimes referred to as "non-orthogonal" frequency hopping systems. One technique used to statistically minimize these "collisions" reduces the total bandwidth used by the transmitters to less than the entire "spread spectrum bandwidth", typically to about 50% of the entire "spread spectrum bandwidth".
In another type of frequency hopping system, the plurality of transmitters are arranged to operate at different carrier frequencies having a preassigned frequency separation and the carrier frequencies change in accordance with a common pseudo-random code. In this system, because all transmitter carrier frequencies are initially non-overlapping and change in accordance with the same pseudo-random code, they will remain non-overlapping during hopping and thus there will be no frequency collisions. Such non-frequency collision systems are sometimes referred to as orthogonal frequency hopping systems.